Products, and in particular food products, are commonly packaged in paperboard boxes or cartons. Examples of such paperboard boxes or cartons include cereal boxes, milk cartons, butter and margarine boxes and beer and soft drink secondary packaging (e.g., paperboard cartons enclosing a plurality of beer or softdrink cans or bottles). For explanatory purposes, the simple term “cartons” may be used throughout this description to refer to the type of paperboard boxes or cartons described above.
The process of forming this type of carton typically begins by printing a continuous web of paperboard material with the particular graphics desired for the package in question. The paperboard material may, for example, have a thickness of between about 0.001 and about 0.040 inch. Before printing, the paperboard material may, for example, be of a brown or grey color. Alternatively, the paperboard material may be bleached or coated so as to exhibit a generally white color. A typical web of paperboard material may, for example, have a length of between about 10,000 and about 30,000 feet and may be wound into a roll format.
To print a web of material, the web of material may be mounted on a reel at one end of a web printing machine. Such a web printing machine typically includes various printing stations, each of the printing stations being adapted to apply a different pattern and color of ink to the web. Each printing station may employ an ink application method such as a gravure or a flexographic method, as is well-known in the web printing industry. As can be appreciated, this type of printing machine will typically have a number of active printing stations equal to the number of graphics colors to be applied to the web. A drying station may also be located after each of the printing stations such that each color pattern will be dried before that portion of the web enters the next printing station.
The end of the web of material may then be threaded through the web printing machine and thereafter rewound onto an output reel at the opposite end of the printing machine. In this manner, the entire web may be fed through the printing machine. Within the printing machine, the graphics for the desired package are repeatedly printed along the web.
After printing is completed, the printed web is removed from the output reel of the printing machine and transferred to a cutting and scoring machine. The cutting and scoring machine cuts the web into a plurality of carton blanks, each of which is registered with the graphics printed in the printing machine. Examples of cutting and scoring machines are generally disclosed in U.S. Pat. No. 4,781,317 and U.S. Pat. No. 5,757,930, both of which are hereby incorporated by reference for all that is disclosed therein. Depending on the design of the particular carton blank, the blank may also be folded or partially folded and glued after completion of the cutting and scoring operation.
The carton blanks may then be shipped to the product filling location. Here, the carton blanks are erected and the desired product inserted. Any necessary final gluing, depending on the type of carton, may also be accomplished at this time. Examples of carton blanks and of cartons formed therefrom are disclosed in U.S. Pat. No. 5,092,516 and U.S. Pat. No. 5,632,404, both of which are hereby incorporated by reference for all that is disclosed therein.
A problem arises when paperboard cartons are used to package products which contain fluids that are capable of permeating the paperboard. Examples of such problematic products include those which are oily or greasy, e.g., products such as butter or margarine. Specifically, oil or grease from such products can penetrate the paperboard of the carton and appear as a stain on the outside of the carton. Such staining detracts from the appearance of the carton and may interfere with the graphics printed thereon. One solution to this problem is to use a modified paperboard material. Such modified paperboard materials are generally treated with a chemical which makes the paperboard material impermeable to oil and grease. Although this type of material works well to prevent oil and grease migration, it is relatively expensive.
Another solution to the problem of oil and grease migration is proposed in U.S. Pat. No. 4,521,492, which is hereby incorporated by reference for all that is disclosed therein. This solution involves coating the paperboard material with a non-leafing metallic ink and a highly pigmented white ink prior to printing graphics onto the paperboard material. The use of metallic inks, however, is disadvantageous for several reasons. At the outset, metallic inks are relatively expensive and their use, thus, prohibitively adds cost to the package. Metallic inks also have a detrimental effect on printability; specifically, it is difficult to obtain good adhesion between a metallic ink layer and a subsequently applied ink layer. Finally, metallic inks are difficult to apply, often, for example, causing plugging of printing machine rollers.
Thus, it would be generally desirable to provide a solution to the problem of grease and oil migration staining in cartons that overcomes the problems associated with prior proposed solutions.